Device and method for controlling fuel cell system having oxygen concentration transient reduction

ABSTRACT

A fuel cell system has a fuel cell generating power using a fuel gas and an oxidizing agent gas serving as materials of the system and a material supply section supplying the materials to the fuel cell. The power generated by the fuel cell is extracted to a load. A device for controlling the fuel cell system has: a material flow calculation section calculating a material flow supplied to the fuel cell so as to cause the fuel cell to generate the power of a required power generation amount; a material reduction limit detection section calculating a limit for reducing the material flow, based on a power generation state of the fuel cell; and a material flow change section controlling the material supply section so as to change the material flow calculated by the material flow calculation section to the limit calculated by the material reduction limit detection section.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 10/556,898, filed Nov. 15, 2005, (National Phase Application ofPCT/JP2004/010079, filed Jul. 8, 2004), which claims priority from priorJapanese Patent Application Nos. 2003-279731, filed Jul. 25, 2003; and2003-324491, filed Sep. 17, 2003, the entire contents of all of whichare incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a fuel cell system which supplies anoxidant gas and a fuel gas to fuel cells, and which thereby causes thefuel cells to generate power, and relates to a device and a method forcontrolling the fuel cell system.

BACKGROUND ART

Recently, a control device for a fuel cell system which supplies ahydrogen gas to a hydrogen electrode of a fuel cell stack and the air toan air electrode of the fuel cell stack, which electrochemically reactsoxygen in the air at air the electrode with hydrogen at the hydrogenelectrode, and which thereby causes the fuel cell stack to generatepower has been studied. Particularly, development of a control devicefor an automotive fuel cell system which supplies the power generated bythe fuel cell stack to a driving motor that generates a vehicle runningtorque is underway.

As one example of the automotive fuel cell stack, a polymer electrolytefuel cell (PEFC) stack is known. The PEFC stack is configured to providea solid polymer membrane between a hydrogen electrode and an airelectrode, and to enable the solid polymer membrane to function as ahydrogen ion conductor. A hydrogen gas is decomposed to hydrogen ionsand electrons in the hydrogen electrode, whereas an oxygen gas, hydrogenions, and electrons are chemically bonded together to generate water inthe air electrode. At this time, the hydrogen ions travel from thehydrogen electrode to the air electrode via the solid polymer membrane.In order so that the hydrogen ions travel via the solid polymermembrane, the solid polymer membrane needs to contain water vapor. Dueto this, the fuel cell system control device needs to humidify the solidpolymer membrane, and keep the solid polymer membrane humid. To thisend, a technique for humidifying the hydrogen gas to be supplied to thefuel cell stack using a humidifier, and for supplying the resultanthydrogen gas to the hydrogen electrode is proposed.

As en effective technique for humidifying the solid polymer membrane, ahydrogen cycling technique for recirculating the hydrogen gas which isnot used by the fuel cell stack but discharged therefrom to the fuelcell stack so as to be recycled by the fuel cell stack is known. In thefuel cell system which adopts the hydrogen cycling, the hydrogen gas issupplied to the hydrogen electrode by an amount slightly larger than arequired hydrogen amount to generate power consumed by a load connectedto an outside of the fuel cell stack, unused hydrogen gas is dischargedfrom an outlet of the hydrogen electrode, and this exhaust hydrogen gas(hereinafter, “cyclic hydrogen”) is returned again to an inlet port ofthe hydrogen electrode. At this moment, the cyclic hydrogen dischargedfrom the fuel cell stack contains much water vapor. Therefore, thecyclic hydrogen is mixed with dry hydrogen supplied from a hydrogentank, a hydrogen mixture is supplied to the hydrogen electrode, and thehydrogen to be supplied to the hydrogen electrode is thereby humidified.

As can be seen, not only a hydrogen flow necessary for power generationbut also an excessive hydrogen flow for humidifying the solid polymermembrane pass through the hydrogen electrode of the fuel cell stack.That is, by supplying a hydrogen flow more than the hydrogen flownecessary for power generation to the hydrogen electrode, all cells thatconstitute the fuel cell stack are enabled to efficiently generatepower.

On the other hand, if only a hydrogen flow corresponding to a requiredpower generation amount is supplied to the hydrogen electrode, thenthere is a probability that hydrogen does not efficiently reach thecells near the outlet port of the hydrogen electrode, and that the powergeneration efficiency of the fuel cell stack is deteriorated. Similarly,not an oxygen flow corresponding to the required power generation amountbut an oxygen flow slightly larger than the oxygen flow corresponding tothe required power generation amount is supplied to the air electrode.Namely, a material stoichiometric ratio that indicates a ratio of aconsumed gas amount to a supplied gas amount is “1” when only thehydrogen flow or the oxygen flow corresponding to the required powergeneration amount is supplied. From viewpoints of humidification andpower generation efficiency, the material stoichiometric ratio isnormally set higher than “1”.

Nevertheless, even if the material stoichiometric ratio is optimum in asystem design phase, it is not always optimum for an operating state ofthe fuel cell stack. Due to this, the material stoichiometric ratio isset higher so as to somewhat include a margin ratio, thereby settingsupply amounts of hydrogen and oxygen to the fuel cell stack to belarger. As a result, materials such as hydrogen are disadvantageouslywasted.

To eliminate waste of the materials, therefore, the material flow isreduced so that the material stoichiometric ratio is closer to “1”. Ifso, however, the supply amounts of hydrogen and oxygen need to bechanged to be sensitive to a change in the operating state (atemperature, a humidity, a material distribution, and the like) of thefuel cell stack, thereby losing robustness. As a result, even with aslight change in the operating state, generated voltage is reduced to alower limit or lower.

To prevent this disadvantage, fuel cell systems intended toappropriately control the material flow are disclosed in Japanese PatentApplication Laid-Open Publication No. 2002-289235, No. 2000-208161, andNo. 2002-164068. Each of the fuel cell systems disclosed thereincontrols the material flow so that a change in a cell voltage of a fuelcell stack falls within an allowable range, controls the material flowso that a generated voltage by the fuel cell stack is equal to or higherthan a lower limit, and increases the material flow when the generatedvoltage of the fuel cell stack falls.

DISCLOSURE OF INVENTION

However, each system detects a changing range of the cell voltage of thefuel cell stack, so that the system is incapable of measuring areduction in the overall generated voltage of the fuel cell stack forthe following reason. Since a voltage to be supplied to a load issupplied as an average voltage, changing ranges of the generatedvoltages of the respective cells cannot be individually measured. As aresult, the system is disadvantageously incapable of detecting that theaverage generated voltage falls even if the irregularity of the cellvoltage of each cell is within the allowable range, or detecting thatthe irregularity of the cell voltage of each cell increases even if theaverage generated voltage is equal to or higher than the lower limit.

Furthermore, to reduce materials as much as possible and enable the fuelcell stack to generate power, a control device for each of the fuel cellsystems needs to accurately grasp a reduction width limit to eachmaterial. Besides, when controlling the supply amount of each materialto be equal to a target amount, the fuel cell systems need to calculatean optimum control amount so as to prevent the supply amount from beingexcessively controlled to exceed the target amount, or from fallingshort of the target amount.

A first aspect of the present invention provides a device and a methodfor controlling a fuel cell system. The fuel cell system has a fuel cellgenerating power using a fuel gas and an oxidizing agent gas serving asmaterials of the fuel cell system and a material supply sectionsupplying the materials to the fuel cell. The power generated by thefuel cell is extracted to a load. The device for controlling the fuelcell system has: a material flow calculation section calculating amaterial flow supplied to the fuel cell so as to cause the fuel cell togenerate the power of a required power generation amount; a materialreduction limit detection section calculating a limit for reducing thematerial flow supplied to the fuel cell, based on a power generationstate of the fuel cell; and a material flow change section controllingthe material supply section so as to change the material flow calculatedby the material flow calculation section to the limit calculated by thematerial reduction limit detection section. The method for controllingthe fuel cell system has: calculating a material flow supplied to thefuel cell so as to cause the fuel cell to generate the power of arequired power generation amount; calculating a limit for reducing thematerial flow supplied to the fuel cell, based on a power generationstate of the fuel cell; and controlling the material supply section soas to change the material flow to the limit.

A second aspect of the present invention provides a fuel cell systemhaving: a fuel cell stack configured to generate power by causinghydrogen and oxygen to electrochemically react with each other; a fuelgas supply device configured to supply a fuel gas that contains thehydrogen, to the fuel cell stack; an oxidizing agent gas supply deviceconfigured to supply an oxidizing agent gas that contains the oxygen, tothe fuel cell stack; a voltage detection section configured to detect avoltage generated by the fuel cell stack; an oxygen concentrationtransient reduction section configured to reduce transiently an oxygenconcentration in a cathode electrode of the fuel cell stack; a voltagevariation detection section configured to detect a voltage variationwhen the oxygen concentration is transiently reduced by the oxygenconcentration transient reduction section; and a voltage stabilizationmaintenance determination section configured to determine whether apresent oxygen utilization ratio is appropriate for maintaining thevoltage of the fuel cell stack stable, based on an output of the voltagevariation detection section.

Other and further features, advantages, and benefits of the presentinvention will become more apparent from the following description takenin conjunction with the following drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a fuel cell system including a controldevice according to a first embodiment of the present invention;

FIG. 2 is a flowchart showing processing procedures for a material flowcontrol processing that is performed by the control device shown in FIG.1;

FIG. 3 is a graph for explaining a relationship among a vibrationamplitude of a generated current or a vibration amplitude of a generatedvoltage, a first predetermined amount, and a second predeterminedamount;

FIG. 4 is a graph for explaining changes in a generated current, agenerated voltage, and a material flow when a constant electric power isextracted from the fuel cell stack by a load;

FIG. 5 is a graph showing a relationship between a cell voltageirregularity and a margin ratio;

FIG. 6 is a graph showing a relationship between an operatingtemperature of the fuel cell stack and the margin ratio;

FIG. 7A is a graph showing a change in the generated voltage with thepassage of time for explaining the change in the generated voltagerelative to change in the material flow;

FIG. 7B is a graph showing changes in the material flow with the passageof time for explaining the change in the generated voltage relative tothe change in the material flow;

FIG. 8 is a graph showing a relationship between the change in thegenerated voltage and sensitivity when the generated voltage and thematerial flow are changed;

FIG. 9 is a block diagram showing one example of functionalconfiguration of the control device shown in FIG. 1 for performing thematerial flow control processing;

FIG. 10 is a block diagram showing another example of the functionalconfiguration of the control device shown in FIG. 1 for performing thematerial flow control processing;

FIG. 11 is a block diagram showing a fuel cell system according to asecond embodiment of the present invention;

FIG. 12 is a flowchart showing one example of concrete operations of thefuel cell system shown in FIG. 11;

FIG. 13A is a graph showing a voltage locus (time response) by atransitional load;

FIG. 13B is a graph showing a change in a voltage measured through avoltage sensor with the passage of time (a voltage difference);

FIG. 13C is a graph showing a relationship between the voltagedifference shown in FIG. 13B and a correction amount for increasing theair flow at transient time;

FIG. 14A is a graph showing a voltage locus (time response) by atransitional load;

FIG. 14B is a graph showing a change in the voltage measured through thevoltage sensor with the passage of time (a voltage irregularity);

FIG. 14C is a graph showing a relationship between the voltageirregularity shown in FIG. 14B and a correction amount for increasingthe air flow at transient time;

FIG. 15A is a graph showing an output of an advancement/delaycompensation filter when a definition equation;

FIG. 15B is a graph showing a step function of the advancement/delaycompensation filter are input to the filter;

FIG. 16 is a flowchart showing another example of specific operations ofthe fuel cell system shown in FIG. 11;

FIG. 17A is a graph showing a current-to-voltage characteristic function(I-V characteristic function) with an oxygen utilization ratio used as aparameter;

FIG. 17B is a graph showing a voltage locus (time response) by atransient load;

FIG. 18A is a graph showing a relationship between a difference betweena target lowest voltage and a lowest voltage and the correction amountfor increasing the air flow at the transient time;

FIG. 18B is a graph showing a relationship between a difference betweena target stationary voltage and a stationary voltage and the correctionamount for increasing the air flow at stationary time;

FIG. 19 is a block diagram showing one example of a voltage variationdetection section shown in FIG. 11;

FIG. 20 is a block diagram showing a voltage stability maintenancedetermination section shown in FIG. 11; and

FIG. 21 is a block diagram showing another example of the voltagevariation detection section shown in FIG. 11.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a fuel cell system and a fuel cell system control device ofeach of various embodiments according to the present invention aredescribed principally with reference to the accompanying drawings FIGS.1 to 21.

First Embodiment

A fuel cell system control device according to a first embodiment of thepresent invention is applied to a fuel cell system shown in FIG. 1, forexample.

[Configuration of Fuel Cell System]

The fuel cell system includes a fuel cell stack 1 which is supplied witha fuel gas which contains a large amount of hydrogen and an oxidizingagent gas which contains oxygen, and which generates power. The fuelcell stack 1 includes a plurality of stacked cell structures, andconstitutes a main power supply of the fuel cell system. Each cellstructure includes a fuel cell structure and separators between whichthe fuel cell structure is held. The fuel cell structure includes asolid polymer electrolyte membrane (PEM) as well as a cathode electrodeand an anode electrode opposed to each other across the solid PEM. Theoxidizing agent gas is supplied to the cathode electrode, and the fuelgas is supplied to the anode electrode.

The fuel cell stack 1 generates power by the following chemicalreaction. In the anode electrode, electrons are emitted from hydrogencontained in the fuel gas and ionized to generate hydrogen ions (H⁺),which hydrogen ions pass through the PEM and reach the cathodeelectrode. In the cathode electrode, the hydrogen ions are bonded withoxygen contained in the oxidizing agent gas to thereby generate water(H₂O).

The fuel cell system shown in FIG. 1 includes a control device 3 whichcontrols supply amounts of materials to the fuel cell stack 1 to therebycontrol power generation of the fuel cell stack 1, and which controls anoperation of a load 2 driven by a generated power from the fuel cellstack 1. The “materials” include herein an oxidizing agent gas and afuel gas. The control device 3 includes a storage section such as a readonly memory (ROM) or the like, which is not shown, and stores a fuelcell control program which describes a series of processing proceduresfor activating the fuel cell system, and for supplying power to the load2. The control device 3 reads signals from various sensors, to bedescribed later, executes the fuel cell control program using a centralprocessing unit (CPU) or the like, which is not shown, issues commandsto respective constituent sections, and thereby controls the respectivesections. If the fuel cell system is installed in a vehicle, the load 2is, for example, a driving motor which generates a driving torque of thevehicle by the power generated by the fuel cell stack 1.

The fuel cell system shown in FIG. 1 includes material supply systemswhich supply materials to the fuel cell stack 1 so that the fuel cellstack 1 generates power. Specifically, the fuel cell system includes ahydrogen supply system for supplying hydrogen to the fuel cell stack 1and an air supply system for supplying oxygen to the fuel cell stack 1.A pure water humidification system, not shown, for humidifying thehydrogen and the air to be supplied to the fuel cell stack 1 using purewater is connected to the hydrogen supply system and the air supplysystem.

The hydrogen supply system includes a hydrogen supply path connected tothe fuel cell stack 1, a hydrogen supply device 4 arranged on thehydrogen supply path, a hydrogen supply valve 5, and a hydrogen supplyactuator 6. The hydrogen supply device 4 includes a hydrogen flowcontrol valve which regulates a hydrogen flow supplied to the fuel cellstack 1. The hydrogen supply system also includes a purge valve 7 and apurge actuator 8 which are arranged on a hydrogen discharge pathconnected to a hydrogen outlet of the fuel cell stack 1, and a cyclichydrogen pump 9 arranged on a hydrogen circulation path connected to ahydrogen inlet of the fuel cell stack 1.

The control device 3 controls an opening of the hydrogen flow controlvalve of the hydrogen supply device 4 in accordance with a powergeneration amount required for the fuel cell stack 1, and controls thehydrogen actuator 6, thereby opening the hydrogen supply valve 5. Thecontrol device 3 further controls a number of revolutions of the cyclichydrogen pump 9 to circulate the hydrogen discharged from the fuel cellstack 1 to the fuel cell stack 1. For example, to discharge gas in ahydrogen electrode so as to eliminate water vapor, impurities, and thelike generated in the fuel cell stack 1, the control device 3 controlsthe purge actuator 8 to open the purge valve 7.

The hydrogen supply system mixes the hydrogen from the hydrogen supplydevice 4 with the cyclic hydrogen from the fuel cell stack 1, supplies ahydrogen mixture to the fuel cell stack 1, and thereby recycles thehydrogen discharged from the fuel cell stack 1. Since the cyclichydrogen is supplied to and discharged from the fuel cell stack 1 whilebeing humidified, the cyclic hydrogen contains much water vapor.Therefore, if the cyclic hydrogen is mixed with the hydrogen from thehydrogen supply device 4, then the humidified hydrogen is supplied tothe fuel cell stack 1, and the solid PEM of the fuel cell stack ishumidified.

The air supply system includes an air supply path connected to the fuelcell stack 1, an air supply device 10 arranged on the air supply path,and an air discharge path connected to an air discharge outlet of thefuel cell stack 1. The air supply device 10 is, for example, acompressor the number of revolutions of which is controlled by a controlsignal from the control device 3.

The control device 3 controls the number of revolutions of thecompressor that constitutes the air supply device 10 in accordance withthe power generation amount required for the fuel cell stack 1.

The load 2 which consumes generated power is connected to the fuel cellstack 1. If the load 2 is, for example, the driving motor, then the load2 includes an inverter which converts a DC power from the fuel cellstack 1 into a desired electric power, and the generated power issupplied to the driving motor through the inverter. In supplying thegenerated power to the driving motor, the control device 3 sets power tobe converted by the inverter, and controls the inverter to extract thegenerated power from the fuel cell stack 1.

The control device 3 includes a voltage sensor 11 and a current sensor12 arranged on a power supply line that connects the fuel cell stack 1to the load 2. The voltage sensor 11 and the current sensor 12 measure avoltage and a current on the power supply line, respectively. Themeasured voltage and current are supplied, as sensor signals, to thecontrol device 3. The control device 3 reads the sensor signals andcontrols an operation of the inverter.

The fuel cell system includes a temperature sensor 13 which detects atemperature of the fuel cell stack 1, and a cell voltage measurementdevice 14 which detects a cell voltage of each cell that constitutes thefuel cell stack 1. When the materials are supplied to the fuel cellstack 1, the control device 3 reads sensor signals from the temperaturesensor 13 and the cell voltage measurement device 14, and carries out amaterial flow control processing for controlling a hydrogen amount andan air amount.

[Material Flow Control Processing]

Referring to FIG. 2, processing procedures for the material flow controlprocessing for controlling flows of the materials (hydrogen and oxygen)by the control device 3 of the fuel cell system shown in FIG. 1 will bedescribed. The material flow control processing is carried out when aload constant control for setting target power for the power to beextracted from the fuel cell stack 1 and for extracting the target powerby the load 2 is exerted.

(A) At a stage S1, the control device 3 calculates a material flownecessary for the fuel cell stack 1 in accordance with a required loadcurrent for the load 2. The material flow includes a necessary hydrogenflow and a necessary oxygen flow. Specifically, the control device 3calculates at least the necessary hydrogen flow based on Faraday's lawusing the required load current. The control device 3 calculates thenecessary oxygen flow necessary for the fuel cell stack 1 to generatethe required load current, from the necessary hydrogen flow. The controldevice 3 sets an oxygen content of the air at, for example, 21 percents.

Specifically, When the required load current of the load 2, that is,target load current is set, the control device 3 calculates thenecessary hydrogen flow to be supplied to the fuel cell stack 1, thatis, supplied hydrogen flow according to Equation (1).(Supplied hydrogen flow)=(Target load current)×(Coefficient)×(Number ofcells)×(Hydrogen stoichiometric ratio)  (1)

In the Equation (1), the coefficient is a value also obtained by theFaraday's law and set at 0.00696 herein. In addition, a unit conversionto NL/min is included in the Equation (1).

When the target load current is set, the control device 3 calculates thenecessary air flow to be supplied to the fuel cell stack 1, that is,supplied air flow according to Equation (2).(Supplied air flow)=(Coefficient)×(Target load current(A))×((Number ofcells)/0.21)×(Air stoichiometric ratio)  (2)

In the Equation (2), the coefficient is a value also obtained by theFaraday's law, and set at 0.00696/2=0.0348 herein. The unit conversionto NL/min is included in the Equation (2).

If only the material flow corresponding to the required load current issupplied, a material distribution in the fuel cell stack 1 is notuniform and a power generation efficiency is thereby deteriorated.Therefore, according to the first embodiment, the stoichiometric ratiois set so as to supply a material flow more than that corresponding tothe required load current to the fuel cell stack 1. This stoichiometricratio is set at an appropriate value by an experiment or the like in adesign phase of the fuel cell system.

However, the stoichiometric ratio is the appropriate value only in thesystem design phase, and the stoichiometric ration set in the systemdesign phase is not always appropriate according to an operating stateor the like of the fuel cell stack 1. Due to this, the control device 3performs the operations as expressed by the Equations (1) and (2), andthen supplies the material flow equal to or higher than thestoichiometric ratio to the fuel cells tack 1 as the material flowaccording to the operating state of the fuel cell stack 1.

Consequently, the control device 3 controls the air supply device 10 tosupply the air flow more than the necessary air flow corresponding tothe required load current to the fuel cell stack 1, and to discharge anexcessive amount of air. In addition, the control device 3 controls thehydrogen supply device 4 and the cyclic hydrogen pump 9 to supply thehydrogen flow more than the necessary hydrogen flow corresponding to therequired load current to the fuel cell stack 1, and to recycle theexcessive hydrogen in the fuel cell stack 1.

(B) At a stage S2, the control device 3 reads a sensor signal from thevoltage sensor 11 or the current sensor 12, and measures a vibrationamplitude of a generated current or a generated voltage aftercontrolling the material flow at the stage S1. The processing procedureswill be continued while referring to an instance of measuring thevibration amplitude of the generated current. However, the sameprocessing procedures can be carried out for an instance of measuringthe vibration amplitude of the generated voltage.

(C) At a stage S3, the control device 3 determines whether the vibrationamplitude of the generated current is equal to or higher than a presetfirst predetermined value. It is noted that vibration of the generatedcurrent hardly occurs while the fuel cell stack 1 stably generates powerunder the control exerted at the stage S1. Even if the materials aresupplied in excess of the material flow corresponding to the requiredload current, the vibration of the generated current hardly occurs.Accordingly, the control device 3 sets the first predetermined value fordetermining whether the material flow can be reduced while hardlygenerating the vibration of the generated current, and compares thevibration amplitude of the generated current measured at the stage S2with the first predetermined value.

(D) When the control device 3 determines that the vibration amplitude ofthe generated current is not equal to or higher than the firstpredetermined value (“NO” at the stage S3), the processing goes to astage S4. When determining that the vibration amplitude of the generatedcurrent is equal to or higher than the first predetermined value (“YES”at the stage S3), the processing goes to a stage S5.

(E) At the stage S5, the control device 3 determines whether thevibration amplitude of the generated current is equal to or higher thana second predetermined value. The “second predetermined value” is set inadvance for determining whether the fuel cell stack 1 does not stablygenerate power due to a smaller material flow. When determining that thevibration amplitude of the generated current is equal to or higher thanthe second predetermined value (“YES” at the stage S5), the processinggoes to a stage S8. When determining that the vibration amplitude of thegenerated current is not equal to or higher than the secondpredetermined value (“NO” at the stage S5), the control device 3finishes the material flow control processing. When the vibrationamplitude of the generated current is not equal to or lower than thefirst predetermined value and not equal to or higher than the secondpredetermined value but between the first and the second predeterminedvalues, then the control device 3 keeps a present material supply state,and prevents hunting of a control for increasing or reducing thematerial flow.

Namely, in periods from a time t0 to time t1 shown in FIG. 3, thecontrol device 3 determines that the vibration amplitude of thegenerated current is equal to or lower than the first predeterminedvalue at the stage S3, and that the material flow can be reduced, andthe processing goes to the stage S4. In periods from the time t1 to atime t2, the control device 3 determines that the vibration amplitude ofthe generated current is equal to or higher than the first predeterminedvalue and lower than the second predetermined value at the stages S3 andS5, finishes the material flow control processing, and keeps the presentmaterial supply state. After the time t2 shown in FIG. 3, the controldevice 3 determines that the vibration amplitude of the generatedcurrent is equal to or higher than the first predetermined value and thesecond predetermined value at the stages S3 and S5, and that thematerials are insufficient, and the processing goes to the stage S8.

(F) At the stage S4, since the control device 3 determines that thevibration amplitude of the generated current is not equal to or higherthan the first predetermined value, the control device 3 sets a limitfor reducing the material flow. When the target power is set, the load 2extracts the same power as the target power, the material flow isreduced as shown in FIG. 4, and the reduced material flow is closer tothe limit to the power generation amount, then the generated voltagefalls and the generated current rises by as much as a reduction in thegenerated voltage. When detecting the rise of the generated current, thecontrol device 3 increases the material flow, whereby the generatedcurrent rises, the generated voltage falls, and the material flow falls.As can be seen, the control device 3 repeatedly executes the control forincreasing the material flow and the control for reducing the materialflow in accordance with the rise and fall of the generated current.

In this way, at the stage S4, the control device 3 sets an increasewidth of the generated current and an reduction width of the generatedcurrent large, sets a reduction width of the material flow so as to havea preset amplitude, and sets the limit for reducing the material flow(hereinafter, “material flow reduction limit”). After the stage S4, theprocessing goes to a stage S6.

(G) At the stage S6, the control device 3 sets the material flowreduction limit set at the stage S4 to be high, and sets a margin ratiofor ensuring that the load 3 can extracts the target power. At thisstage, the control device 3 reads the sensor signal from the temperaturesensor 13, and sets the margin ratio higher as the operating temperatureof the fuel cell stack 1 is lower. The control device 3 also reads thesensor signal from the cell voltage measurement device 14, sets themargin ratio higher as a difference (irregularity) among respective cellvoltages is larger. Besides, the control device 3 sets the margin ratiohigher as the amplitude of the generated current measured at the stageS2 is higher.

When the material flow is reduced to near the material flow reductionlimit, the generated voltage easily exceeds the lower limit even with aslight change in an operating environment of the fuel cell stack 1. As aresult, the robustness of the fuel cell stack 1 to the operatingenvironment is degraded. If an operating temperature of the fuel cellstack 1 is low and power generation efficiency is lowered, or if thecell voltage difference (irregularity) among difference cells is largeand an arrival distribution of materials is present in the fuel cellstack 1, the robustness of the fuel cell stack 1 to the operatingenvironment is degraded more conspicuously. In such a state, thegenerated voltage easily exceeds the lower limit by a slight change in aregion to which these various sensors cannot be attached or a slightchange which cannot be measured by the sensors. Accordingly, the controldevice 3 sets the margin ratio for the material flow reduction limit sothat the generated voltage does not exceed the lower limit, and sets theactual material reduction limit slightly higher.

Specifically, the control device 3 stores map data shown in FIGS. 5 and6 calculated by experiments or the like in advance. The control device 3sets the margin ratio to be exponentially higher by reference to the mapdata shown in FIG. 5 as the irregularity of each cell voltage is larger.The control device 3 sets the margin ratio to be exponentially lower byreference to the map data shown in FIG. 6 as the operating temperatureof the fuel cell stack 1 is higher. The control device 3 also stores mapdata, not shown, which indicates a change in the margin ratio relativeto the vibration amplitude of the generated current or that of thegenerated voltage, and sets the margin ratio based on this map data.

According to the first embodiment, the instance in which the controldevice 3 sets the margin ratio based on the operating temperature of thefuel cell stack 1, the irregularity of each cell voltage, and thevibration amplitude of the generated current is described. However, thepresent invention is not limited to this instance. For example, thecontrol device 3 may set the margin ratio using parameters for one ofthe operating temperature of the fuel cell stack 1, the irregularity ofeach cell voltage, and the vibration amplitude of the generated current.In addition, the control device may set the margin ratio first based ona certain parameter, and change the set margin ratio based on the otherparameter. Further, a humidity sensor which detects a humid state of thesolid PEM that constitutes the fuel cell stack 1 may be provided at eachof the hydrogen electrode and the air electrode, the control device 3may read a sensor signal from the humidity sensor and set the marginratio higher as the humidity is lower.

(H) At a stage S7, the control device 3 calculates a material reductionamount by which the material flow currently supplied to the fuel cellstack 1 is reduced in a control cycle of increasing or reducing thematerial flow.

When the material flow is changed as shown in FIG. 7B, the generatedvoltage tends to have great change when the generated voltage is low, asshown in FIG. 7A, and to have small change when the generated voltage ishigh. That is, even with the same change of the material flow, a voltagevariation differs according to the generated voltage. When the materialflow is equal to or higher than a predetermined amount A1, the change ofthe material flow hardly contributes to the voltage variation.Accordingly, even if the material flow is set at the predeterminedamount A1 or higher, the generated voltage does not rise, so that thematerials are wasted.

Therefore, the control device 3 stores “a materialvoltage-to-sensitivity function” which indicates a relationship of achange in material flow (sensitivity) to the generated voltage, i.e.,how much the generated voltage is changed from the present generatedvoltage based on power generation characteristics of the fuel cell stack1 shown in FIGS. 7A and 7B. At the stage S7, the control device 3predicts a change in generated voltage when the material flow is changedusing the material voltage-to-sensitivity function, and calculates thereduction amount of the material flow. The control device 3 controls theair supply device 10, the hydrogen supply device 4, and the cyclichydrogen pump 9 based on the calculated material flow reduction amountto thereby reduce the material flow.

(I) When the vibration amplitude of the generated current is equal to orhigher than the second predetermined value (“YES” at the stage S5), theprocessing goes to the stage S8, at which the control device 3 uses thematerial voltage-to-sensitivity function similarly to the stage S7. Atstage S8, the control device 3 increases the material flow, unlike atthe stage S7 which reduces the material flow. This is because thevibration amplitude of the generated current is high. Even if thecontrol device 3 reduces the material flow at the stage S7 and thereduction amount of the material flow is too large, then the controldevice 3 controls the air supply device 10, the hydrogen device 4, andthe cyclic hydrogen pump 9 to recover the material flow based on thematerial flow at the stage S8.

At the stage S2, the control device 3 determines whether the materialflow is increased or reduced using the vibration amplitude of thegenerated current or that of the generated voltage. However, the presentinvention is not limited thereto. For example, when an oxygenconcentration sensor is provided at the outlet port of the air electrodeand an oxygen concentration detected by the oxygen concentration sensoris high, the control device 3 may reduce the material flow. When theoxygen concentration is low, the control device 3 may increase thematerial flow. Alternatively, when an air flow sensor is provided at theoutlet port of the air electrode and an air flow detected by the airflow sensor is high, the control device 3 may reduce the material flow.When the air flow is low, the control device 3 may increase the materialflow.

As shown in FIG. 9, the control device 3 which performs the materialflow control processing shown in FIG. 2 includes a material flowcalculation section 21 which calculates the material flow based on theoperating state of the fuel cell stack 1 at the stages S1 to S3 and thestage S5, a material reduction limit detection section 22 which sets thematerial flow reduction limit at the stage S4, and a material flowchange section 23 which calculates the material flow and which feeds acontrol signal to the respective sections at the stages S7 and S8.

Alternatively, the control device 3 may include functional blocks shownin FIG. 10 instead of the functional blocks shown in FIG. 9. Namely, thecontrol device 3 may include a load control section 24 which controlsthe load 2 so as to extract a target voltage from the fuel cell stack 1,a vibration amplitude detection section 25 which detects the vibrationamplitude of the generated current or that of the generated voltagebased on the generated current or the generated voltage at the stage S2,and a margin ratio calculation section 26 which calculates the marginratio based on the vibration amplitude of the generated current or thatof the generated voltage, the irregularity of each cell voltage, and theoperating temperature of the fuel cell stack 1 or the humidity of thesolid PEM at the stage S6.

Advantages of First Embodiment

As stated above in detail, the fuel cell system according to the firstembodiment of the present invention calculates the material flowreduction limit to the reduction of the material flow supplied to thefuel cell stack 1 based on the power generation state of the fuel cellstack 1, and controls the material flow. The fuel cell system canthereby cause the fuel cell stack 1 to generate power while effecting asaving in the materials as large as possible in the present operatingstate of the fuel cell stack 1, and can supply the power to the load 2.Therefore, it is possible to avoid the waste of the materials suppliedto the fuel cell stack 1, and to cause the fuel cell stack 1 to stablygenerate power.

The fuel cell system according to the first embodiment selects thechange of the generated current or that of the generated voltageextracted from the fuel cell stack 1 by the load 2, and calculates thematerial flow reduction limit based on the change of the generatedcurrent or that of the generated voltage. By reducing the material flowaccording to the material flow reduction limit, the vibration amplitudeof the generated current or that of the generated voltage is increasedas shown in FIG. 3. It is, therefore, possible to prevent the powergeneration of the fuel cell stack 1 from being made unstable by thechange of the generated current or that of the generated voltage.Namely, by setting the material flow reduction limit high so as not toincrease the vibration amplitude of the generated current or that of thegenerated voltage, the material flow can be reduced.

When the load 2 is controlled to extract the constant power from thefuel cell stack 1 to increase or reduce the material flow as shown inFIG. 4, the fuel cell system according to the first embodimentcalculates the material flow reduction limit based on the vibrationamplitude of the generated current or that of the generated voltage. Itis thereby possible to set the material flow at the material flowreduction limit at an increase point after reducing the generatedcurrent or the generated voltage, and to ensure controlling the materialflow to be set at the material flow reduction limit.

The fuel cell system according to the first embodiment calculates themargin ratio which indicates a degree of increasing the material flowreduction limit based on the vibration amplitude of the generatedcurrent or that of the generated voltage, and on the operating state ofthe fuel cell stack 1. The material flow can be set higher than thematerial flow reduction limit, and the robustness of the fuel cell stack1 to the environmental change that influences the power generation ofthe fuel cell stack 1 can be kept high. Therefore, even if an erroroccurs to the setting of the material flow reduction limit due to anoise or the like in the detection of, for example, the operating stateof the fuel cell stack 1, it is possible to prevent the material flowfrom being equal to or lower than the actual limit. In addition, even ifa delay occurs to the actual change in the material flow by a delay ofcontrol when increasing the material flow from the lower limit, it ispossible to ensure preventing the material from being equal to or lowerthan the actual limit.

The fuel cell system according to the first embodiment sets the marginratio higher as the vibration amplitude of the generated current or thatof the generated voltage is higher. By so setting, even if a delayoccurs to a change in the actual material flow when the material flow isto be greatly reduced, it is possible to prevent the actual materialflow from being equal to or lower than the limit. The fuel cell systemaccording to the first embodiment sets the margin ratio higher as theoperating temperature of the fuel cell stack 1 is lower. By so setting,when the operating temperature of the fuel cell stack 1 is low and thepower generation state of the fuel cell stack 1 tends to be unstable, itis possible to prevent the actual material flow from being equal to orlower than the limit. Furthermore, by setting the margin ratio higher asthe irregularity of each cell voltage is larger, it is possible toprevent the generated voltage from being equal to or lower than thelower limit even if the material distribution state in the fuel cellstack 1 is irregular. Besides, by setting the margin ratio higher as thehumidity of the solid PEM is lower, the generated voltage can beprevented from being equal to or lower than the lower limit even if thehumidity of the solid PEM is low and the generated voltage tends to beequal to or lower than the lower limit by a slight environmental change.

The fuel cell system according to the first embodiment calculates thematerial flow to be increased or reduced based on the materialvoltage-to-sensitivity function which indicates the generated voltagevariation relative to the change in the material flow according to thepresent generated voltage if the material flow is to be changed based onthe material flow reduction limit. By so calculating, it is possible toprevent the generated voltage from being greatly changed according tothe change in the material flow and exceeding the limit, and therebypreventing the material flow from being excessively changed.

The fuel cell system according to the first embodiment changes thematerial flow using the fuel voltage-to-sensitivity function accordingto the present generated voltage. It is thereby possible to predict achange in the generated voltage when the material flow is changed, andprevent excessive increase or reduction of the materials. In addition,it is possible to prevent the generated voltage from being greatlychanged according to the change in the material flow and exceeding thelimit, and thereby prevent the material flow from being excessivelychanged. At the same time, it is possible to prevent an insufficientchange in the material flow when the material flow is to be reduced.

When the air flow discharged from the fuel cell stack 1 or the oxygenconcentration in the discharged air is lower than the predeterminedvalue by reducing the material flow, the fuel cell system according tothe first embodiment sets the reduced material flow as the material flowreduction limit. It is thereby possible to set the material flowreduction limit in accordance with changes in the air and oxygenconsumed by a power generation reaction that actually occurs to the fuelcell stack 1. Thus, the material flow reduction limit can be easilyaccurately set.

Namely, when almost all oxygen is consumed by the power generationreaction of the fuel cell stack 1 by the reduction of the air flow,components of the exhaust air are almost nitrogen and the oxygenconcentration of the exhaust air is reduced. In order to keep the oxygendistribution in the fuel cell stack 1 uniform and to efficiently triggerthe power generation reaction, it is necessary to supply the air inexcess of the required power generation amount to the fuel cell stack 1,and to keep the oxygen concentration of the exhaust air to be equal toor higher than the predetermined value. The fuel cell system accordingto the first embodiment can set the material flow reduction limit andcontrol the material flow using these power generation reactioncharacteristics.

The fuel cell system according to the first embodiment increases orreduces the material flow so that the vibration amplitude of thegenerated current or that of the generated voltage extracted from thefuel cell stack 1 is equal to or lower than the second predeterminedvalue, and increases or reduces the material flow so that the air flowdischarged from the fuel cell stack 1 or the concentration of oxygencontained in the exhaust air falls within the predetermined range. It isthereby possible to reduce the material flow to be equal to or lowerthan the material flow reduction limit and thereby save the materials,and to prevent the generated voltage from being equal to or lower thanthe lower limit without reducing the material flow to be equal to orlower than the material flow reduction limit.

When the material flow is reduced, the generated voltage falls. Thelower limit voltage that indicates a width by which the materials can bereduced depends on the operating state. The fuel cell system accordingto the first embodiment, therefore, detects the lower limit voltage whenreducing the material flow. The materials can be thereby reduced as muchas possible, and the waste of the materials can be eliminated.

Moreover, when the material flow is increased to restore the generatedvoltage, the fuel cell system according to the first embodiment givesregard to characteristics of the fuel cell stack 1 that the increasewidth of the generated voltage is smaller as the material flow isincreased and the generated voltage rises. It is thereby possible toavoid the waste of the materials without excessively increasing orexcessively reducing the material flow. The reason is as follows. Whenthe generated voltage is low, the generated voltage has great changeaccording to the increase amount of the material flow. However, when thegenerated voltage is high to some extent, the increase width of thegenerated voltage is saturated relative to the increase amount of thematerial flow.

As can be understood, the fuel cell system according to the firstembodiment calculates the limit for reducing the material flow to besupplied to the fuel cell stack 1, and controls the material flow.Therefore, the fuel cell system according to the first embodimentenables the fuel cell stack to generate power while effecting the savingin the materials as large as possible in the present operating state ofthe fuel cells, and the power can be supplied to the load 1. Accordingto the first embodiment of the present invention, therefore, the fuelcell system control device capable of eliminating the waste of materialssupplied to the fuel cells, and ensuring that the fuel cells stablygenerate power can be provided.

Second Embodiment

In a second embodiment of the present invention, a fuel cell systemwhich reduces an air stoichiometric ratio, which stabilizes a generatedvoltage, and which thereby improves a fuel consumption performance willbe described.

The fuel cell system includes an anode electrode supplied with ahydrogen gas, a cathode electrode supplied with an oxidizing agent gassuch as the air, and an oxidizing agent gas supply device (e.g., airsupply device) which supplies the oxidizing agent gas to the cathodeelectrode. The fuel cell system triggers a chemical reaction betweenoxygen at the cathode electrode and hydrogen at the anode electrode, andthereby generates power.

In order to improve the fuel consumption of the fuel cell system, it isnecessary to reduce power consumption of the air supply device. Namely,it is necessary to operate the system while reducing an air flow to besupplied as much as possible, and reducing the air stoichiometric ratio.Therefore, when the load connected to the fuel cell is transientlyincreased, an amount of oxygen used by the fuel cell may possibly betransiently increased and the air flow may possibly become insufficient.Specifically, voltage abnormally falls or the irregularities ofrespective cell voltages increase by a slight state change within thefuel cell, generated voltage is made unstable, and an operation of thefuel cell cannot be continued. Since the slight state change within thefuel cell cannot be detected by a sensor or the like, it is impossibleto detect whether the generated voltage is made unstable beforehand, andtiming of increasing the air stoichiometric ratio delays.

To improve the fuel consumption of the fuel cell stack, it is thusnecessary to reduce the air stoichiometric ratio and stabilize thegenerated voltage.

As a method for detecting whether the generated voltage is stable, amethod for determining an abnormality by voltage variation when the airstoichiometric ratio is reduced down to an operation limit is known(Patent Literature 1). According to the Patent Literature 1 (JapanesePatent Application Laid-Open Publication No. H08-007911), a state ofreducing the air stoichiometric ratio is a state in which the oxygenconcentration of the cathode electrode of the fuel cell is reduced. Themethod uses a function that a voltage drop of a defective cell or a cellhaving an unsatisfactory oxygen distribution is amplified in this stateand appears conspicuously.

However, with a view of improving the fuel consumption, the systemnormally operates while setting the air stoichiometric ratio to near anoperating limit. If so, the method disclosed in the Literature 1 cannotdetect whether the generated voltage is unstable. As a result, a defectmay possibly occur that the generated voltage abnormally falls or theirregularities of the respective cell voltages increase and theoperation cannot be continued.

As shown in FIG. 11, the fuel cell system according to the secondembodiment includes the anode electrode supplied with the fuel gas whichis a material gas, and the cathode electrode supplied with the oxidizingagent gas which is also a material gas. The fuel cell system alsoincludes a fuel cell stack 34 which generates power by theelectrochemical reaction of the oxygen at the cathode electrode with thehydrogen at the anode electrode, a fuel gas supply device (hydrogensupply tank) 32 which supplies the fuel gas to the fuel cell stack 34, ahydrogen pressure regulator 33 which regulates a pressure of the fuelgas supplied from the hydrogen supply tank 32, a hydrogen circulationpump 35 arranged on a hydrogen circulation path for returning unusedfuel gas discharged from an outlet port of the anode electrode to aninlet port of the anode electrode for recycle, an oxidizing agent gassupply device (air supply device) 32 which supplies the oxidizing agentgas to the fuel cell stack 34, a load device 30 connected to the fuelcell stack 34 and consuming the power generated by the fuel cell stack34, a voltage detection section (voltage sensor) 36 which detects thepower generated by the fuel cell stack 34, a current detection section(current sensor) 37 which detects a current flow carried from the fuelcell stack 34 to the load device 39, a cell voltage sensor 38 whichdetects voltages of a plurality of fuel cells (hereinafter, “cells”)that constitute the fuel cell stack 34, an air pressure regulation valve40 which detects the pressure of the cathode electrode, a purgingregulation valve 41 which discharges nitrogen accumulated in a hydrogencirculation system, and a control section 42 which controls an overalloperation of the fuel cell system.

The control section 42 includes an oxygen concentration transientreduction section 43 which transiently reduces the oxygen concentrationat the cathode electrode, a voltage variation detection section 44 whichdetects a voltage variation when the oxygen concentration is transientlyreduced by the oxygen concentration transient reduction section 43, anda voltage stability maintenance determination section 45 whichdetermines whether a present oxygen utilization ratio is appropriate formaintaining the voltage of the fuel cell stack 34 stable based on anoutput of the voltage variation detection section 44. The voltage sensor36, the current sensor 37, and the cell voltage sensor 38 transfer avoltage value, a current value, and a voltage value to the controlsection 42, respectively. The control section 42 controls operations ofthe respective constituent elements of the fuel cell system includingthe air supply device 31, the load device 39, and the air pressureregulation valve 40 through a control signal CTR.

As shown in FIG. 19, the voltage variation detection section 44 includesat least a lowest voltage measurement section 55 which measures thevoltage when the oxygen concentration is transiently reduced by theoxygen concentration transient reduction section 43 and the voltage ofthe fuel cell stack 34 falls to the lowest voltage, aprior-to-extraction voltage storage section 56 which stores the voltagebefore the load is extracted, and a first voltage difference detectionsection 57 which detects a difference between the lowest voltage and theprior-to-extraction voltage.

The voltage variation detection section 44 also includes a cell voltagemeasurement section 58 which measures the respective cell voltages ofthe fuel cell stack 34 right after the oxygen concentration istransiently reduced by the oxygen concentration transient reductionsection 43, and an irregularity detection section 59 which detects theirregularity of each cell voltage based on an output of the cell voltagemeasurement section 58.

As shown in FIG. 20, the voltage stability maintenance determinationsection 45 includes a transient-time correction section 61 whichdetermines that the present oxygen utilization ratio is inappropriatefor maintaining the voltage of the fuel cell at the transient timestable if the output of the voltage variation detection section 44exceeds a predetermined value, and a stationary-time correction section62 which reduces the oxygen utilization ratio at the transient timesimilarly to the transient-time correction section 61.

As shown in FIG. 19, the voltage variation detection section 44 furtherincludes an erroneous detection prevention section 60 which blows offwater remaining in the cathode electrode and which discharges the waterto the outside. The erroneous detection prevention section 60 increasesa flow of the oxidizing agent gas or a pressure difference between theinlet and the outlet of the cathode electrode, and thereby accelerates aflow velocity of the oxidizing gas within the cathode electrode when theoutput of the voltage variation detection section 44 deviates from apredetermined limit. This operation will be referred to as “waterblowoff purging”.

The transient-time correction section 61 and the stationary-timecorrection section 62 include target flow calculation sections 63 a and63 b each calculating a target air flow based on a target powergeneration amount and a target oxygen utilization ratio, and oxygenutilization ratio correction sections 64 a and 64 b each applying anadvancement/delay compensation filter to the target air flow calculatedby the target flow calculation section 63 a or 63 b, changing parametersfor the advancement/delay compensation filter, and correcting the oxygenutilization ratio, respectively.

A part of the hydrogen supplied to the anode electrode of the fuel cellstack 34 is discharged from the outlet of the anode electrode withoutbeing used for power generation. This discharge hydrogen is returned tothe inlet of the anode electrode for recycling. Thereafter, the hydrogenreturned to the inlet of the anode electrode for recycling will bereferred to as “cyclic hydrogen”. Hydrogen gas in which the hydrogensupplied from the hydrogen supply tank 32 is mixed with the cyclichydrogen is supplied to the anode electrode. Since the cyclic hydrogencontains much water vapor, the cyclic hydrogen is mixed with the dryhydrogen supplied from the hydrogen supply tank 32 to humidify thehydrogen supplied to the anode electrode. A hydrogen mixture of thehydrogen supplied from the hydrogen supply tank 32 and the cyclichydrogen is supplied to the anode electrode, and a solid PEM of the fuelcell stack 34 is sufficiently humidified. Further, a hydrogencirculation pump 35 is used for circulating the cyclic hydrogen.

According to the second embodiment, a compressor is employed as the airsupply device 31. In the load device 39, an inverter is connected to thefuel cell stack 34, converts energy into power, and supplies the powerto a driving motor. If the fuel cell system is applied to a vehicle, thedriving motor is used to generate a driving power for operating thevehicle. In addition, the power generation amount is set to the loaddevice 39, and a load current from the fuel cell stack 34 is extractedby the load device 39. The control section 42 is composed of a centralprocessing unit (CPU) of an ordinary computer system and a peripheralinterface thereof.

As the fuel cell stack 34 for vehicle, a solid polymer membrane fuelcell stack is used. The solid polymer membrane fuel cell stack 34includes a solid polymer membrane arranged between the anode electrodeand the cathode electrode, and the solid polymer membrane functions as ahydrogen ion conductor. The hydrogen gas is decomposed to hydrogen ionsand electrons in the anode electrode of the fuel cell stack 34, whereasthe oxygen gas, the hydrogen ions, and the electrons are chemicallybonded together to generate water in the cathode electrode thereof. Atthis time, the hydrogen ions travel to the cathode electrode via thesolid polymer membrane. In order so that the hydrogen ions travel viathe solid polymer membrane, the solid polymer membrane needs to containwater vapor. In order to humidify the solid polymer membrane, thehydrogen supplied to the fuel cells is humidified and the humidifiedhydrogen is supplied to the anode electrode. To humidify the hydrogen,the hydrogen cycling technique for recirculating the hydrogen gas whichis not used by the fuel cell stack 34 to the fuel cell stack 34 so as tobe recycled by the fuel cell stack 34 is used. Specifically, thehydrogen is supplied to the anode electrode by an amount slightly largerthan a required hydrogen amount to generate power consumed by the loadconnected to the outside of the fuel cell stack 34, the unused exhausthydrogen gas (cyclic hydrogen) discharged from the outlet port of thecathode electrode is returned again to the inlet port of the anodeelectrode for recycling. Therefore, the hydrogen flow, which includesthe hydrogen flow necessary for power generation and the excessivecyclic hydrogen flow, passes through the anode electrode of the fuelcell stack 34.

By supplying the hydrogen flow larger than the hydrogen flow necessaryfor power generation to the anode electrode, all the cells thatconstitute the fuel cell stack 34 are enabled to efficiently generatepower. If only the hydrogen flow necessary for power generation issupplied to the anode electrode, hydrogen does not efficiently reach thecells near the outlet port of the anode electrode, and the powergeneration efficiency is deteriorated. Similarly, not an oxygen flownecessary for power generation but a slightly excessive oxygen flowslightly is supplied to the cathode electrode. Therefore, by supplyingnot only the oxygen flow necessary for power generation but the slightlyexcessive oxygen flow to the cathode electrode, all the cells thatconstitute the fuel cell stack 34 are enabled to efficiently generatepower.

A ratio of actually supplied oxygen flow/hydrogen flow to oxygenflow/hydrogen flow used for power generation is referred to as an oxygenstoichiometric ratio/hydrogen stoichiometric ratio. If only the hydrogenor oxygen used for power generation is supplied, the hydrogenstoichiometric ratio or the oxygen stoichiometric ratio is “1”.Normally, however, the ratio is set higher than “1” for the reasonsstated above.

When the load is transiently increased, the hydrogen flow necessary forthe reaction of each fuel cell is transiently increased. Since thehydrogen is supplied from the hydrogen supply tank 32, a supply delay ofthe hydrogen occurs. Likewise, the oxygen amount necessary for thereaction of each fuel cell is transiently increased. Since the oxygen issupplied from the air supply device 31 and larger in mass than thehydrogen, a supply delay of the oxygen is more conspicuous than that ofthe hydrogen. Due to this, when the load is transiently increased, thereis a high probability of shortage of the air.

Referring next to FIGS. 12 to 15, one example of specific operations ofthe fuel cell system shown in FIG. 11 will next be described.

(a) At a stage S10, a response of the load extracted from the fuel cellstack 34 is selected. Namely, the load is extracted by a faster responsethan an air supply response, and a low oxygen concentration state istransiently created. Specifically, the oxygen concentration transientreduction section 43 increases the load extracted from the fuel cellstack 34 by the faster response than the air supply response in whichthe air arrives from the air supply device 31 to the cathode electrode,thereby transiently reducing the oxygen concentration of the cathodeelectrode of the fuel cell stack 34. It is noted that the air supplyresponse can be known from data on an experiment conducted in advance.When the load extraction response is too fast, the transient voltagedrop often falls below the lower limit. Considering this, as the load isheavier, the load extraction response is set slower.

(b) At a stage S20, the voltage variation detection section 44 measuresa voltage drop (voltage difference) 51 through the voltage sensor 36when the oxygen concentration is transiently reduced, that is, when theload is transiently extracted as shown in FIG. 13A. Specifically, asshown in FIG. 13B, the lowest voltage measurement section 55 measures avoltage when the oxygen concentration transient reduction section 43transiently reduces the oxygen concentration and the voltage of the fuelcell stack 34 falls down to a lowest voltage 50. The prior-to-extractionvoltage storage section 56 stores a voltage 52 before extracting theload. The first voltage difference detection section 57 detects adifference (voltage difference 51) between the lowest voltage 50 and thevoltage 52 before extracting the load.

(c) At a stage S30, the cell voltage measurement section 58 measureseach cell voltage through the cell voltage sensor 38 right after theoxygen concentration transient reduction section 43 transiently reducesthe oxygen concentration, that is, the load is transiently extracted asshown in FIG. 14A, the voltage falls down to the lowest voltage 50. Theirregularity measurement section 59 detects an irregularity 54 of eachcell voltage based on an output of the cell voltage sensor 38 as shownin FIG. 14B.

(d) At a stage S40, it is determined whether the voltage drop (voltagedifference) 51 measured at the stage S20 is smaller than a predeterminedlower limit. When the voltage drop (voltage difference) 51 is smallerthan the predetermined lower limit (“YES” at the stage S40), theprocessing goes to a stage S50. When the voltage drop (voltagedifference) 51 is equal to or greater than the predetermined lower limit(“NO” at the stage S40), the processing goes to a stage S90. It is notedthat the predetermined lower limit is set higher than an operation limitlower limit.

(e) At the stage S50, it is determined whether the irregularity 54 ofeach cell voltage measured at the stage S30 falls within a predeterminedlimit. When the irregularity 54 of each cell voltage falls within thepredetermined limit (“YES” at the stage S50), the processing goes to astage S95. When the irregularity 54 exceeds the predetermined limit(“NO” at the stage S50), the processing goes to a stage S90. It is notedthat the predetermined limit is set smaller than an operation limitirregularity.

(f) When the determination result at the stage S40 is “NO”, theerroneous detection prevention section 60 determines that a situation inwhich the voltage drop (voltage difference) 51 is grater than theoperation limit lower limit but below the predetermined lower limitsignifies that the voltage falls due to some factor other than shortageof the air. When the determination result at the stage S50 is “NO”, theerroneous detection prevention section 60 determines that a situation inwhich the irregularity 54 is smaller than the operation limitirregularity but exceeds the predetermined limit signifies that theirregularity of each cell voltage increases due to some factor otherthan the shortage of the air. The most frequent factor is occurrence ofa phenomenon that generated water remaining in an air channel of eachfuel cell and closes the channel. According to the second embodiment,the remaining water is blown off so as to eliminate this phenomenonfirst. At the stage S90, therefore, the erroneous detection preventionsection 60 determines whether the water blowoff purging is continuouslyexecuted a predetermined number of times. When the voltage does notrecover even after the water blowoff purging is continuously executedthe predetermined number of times (“YES” at the stage S90), theerroneous detection prevention section 60 determines that an abnormalityoccurs and the processing goes to a stage S110, at which the erroneousdetection prevention section 60 issues “an abnormality alarm”.

(g) When the water blowoff purging is not continuously executed thepredetermined number of times (“NO” at the stage S90), the processinggoes to a stage S100, at which the erroneous detection preventionsection 60 performs water blowoff purging for blowing off the waterremaining in the air channel of each fuel cell. The water blowoffpurging according to the second embodiment is performed by increasing apressure difference between the inlet and the outlet of the cathodeelectrode. The cathode electrode pressure control valve (air pressureregulation valve) 40 is operated to set the pressure of the cathodeelectrode high. Thereafter, the opening of the cathode electrodepressure control valve 40 is increased at a burst to release thepressure. At this time, the air passes through the cathode electrode ofthe fuel cell stack 34 at a burst and is output together with the water.Next, at a stage S105, a variable indicating whether the water blowoffpurging is continued the predetermined times is counted up.

At a stage S95, a water blowoff counter is reset under conditions thatthe voltage drop is within the predetermined value and the irregularityof each cell voltage is within the predetermined limit, that is, thewater blowoff operation is unnecessary.

(h) At a stage S60, the transient-time correction section 61 determinesthat the present oxygen utilization ratio is inappropriate to maintainthe voltage of the fuel cell at the transient time stable, and reducesthe oxygen utilization ratio at the transient time. Specifically, thetransient-time correction section 61 corrects the air flow at thetransient time to be increased in accordance with a difference betweenthe voltage when the load is transiently extracted and the voltage fallsdown to the lowest voltage and the voltage before extracting the load.As shown in FIG. 13C, the air flow at the transient time is corrected tobe increased as the voltage difference 51 is larger. More specifically,the target flow calculation section 63 a calculates a target air flowbased on a target power generation amount and a target oxygenutilization ratio. The oxygen utilization ratio correction section 64 aapplies the advancement/delay compensation filter as shown in FIG. 15Ato the target air flow calculated by the target flow calculation section63 a, changes parameters for the advancement/delay compensation filter,and corrects the oxygen utilization ratio. As shown in FIG. 15B, the airflow at the transient time can be increased by setting an advancementcompensation amount (a1/c) large.

The target air flow is calculated based on the target load current andthe air utilization ratio stored in the ROM in advance as expressedbelow. “Coefficient” is a value obtained by multiplying a Faraday'sconstant and [NL/min] by a unit conversion coefficient, that is, 0.0348.The Faraday's constant is an electric charge held by a group ofparticles of one mole having an elementary electric charge, and F=Ne. Nis an Avogadro constant.Target load current[A]=(Target load[kW])÷(Present voltage[V])Necessary oxygen amount[NL/min]=Coefficient×(Target loadcurrent[A])×(Number of cells)Target air flow[NL/min]=(Necessary oxygen amount)×(Air utilizationratio/0.21)

(i) At a stage S70, the stationary-time correction section 62 determinesthat the present oxygen utilization ratio is inappropriate to maintainthe fuel cell voltage at the stationary time stable, and reduces thestationary-time oxygen utilization ratio. Specifically, thestationary-time correction section 62 corrects the air flow at thestationary time to be increased in accordance with the irregularity ofeach cell voltage right after the load is transiently extracted and thevoltage falls down to the lowest voltage. As shown in FIG. 14C, the airflow at the stationary time is corrected to be increased as theirregularity 54 is larger. More specifically, the target flowcalculation section 63 b calculates the target air flow based on thetarget power generation amount and the target oxygen utilization ratio.The oxygen utilization ratio correction section 64 b applies theadvancement/delay compensation filter as shown in FIG. 15A to the targetair flow calculated by the target air calculation section 63, changesthe parameters for the advancement/delay compensation filter, andcorrects the oxygen utilization ratio. As shown in FIG. 15B, the airflow at the stationary time can be increased by setting a stationarycompensation amount (b/d) large.

Further, as shown in FIG. 15B, by adjusting a delay compensationparameter (c) for the advancement/delay compensation filter, time untilthe air flow is stabilized to a stationary value after the air flow atthe transient time is increased by an advancement function can beadjusted, and the air flow can be stabilized to the stationary value bytaking a long time, so that a state of a low oxygen utilization ratiocan be set long. When a transient load variation frequency is high, theair flow can be stabilized to the stationary value by taking a long timeand the state of the low oxygen utilization ratio can be set long, sothat the voltage drop at the transient time can be reduced.

As stated above, according to the second embodiment of the presentinvention, the oxygen concentration transient reduction section 43transiently creates the low oxygen concentration state. The voltagevariation detection section 44 measures the voltage variation while theoxygen concentration is low. The voltage stabilization maintenancedetermination section 45 determines whether the present oxygenutilization ratio is appropriate to maintain the fuel cell voltagestable before the voltage becomes unstable. When the low oxygenconcentration state is created, the voltage drop of the cell having anunsatisfactory oxygen distribution is amplified. Therefore, by measuringthe voltage variation in the low oxygen concentration state, a potentialfactor for making the voltage unstable can be determined beforehand.

The oxygen concentration transient reduction section 43 can extract theload from the fuel cell stack 34 by the response faster than theresponse of supplying the air arrives from the air supply device 31 tothe cathode electrode of the fuel cell stack 34, and transiently createthe low oxygen concentration state.

The voltage variation detection section 44 can measure the voltagevariation from the difference between the lowest voltage at the time theload is transiently extracted and the voltage before the load isextracted. Based on this voltage variation, it is possible to determinewhether the transient air flow is appropriate.

The voltage variation detection section 44 can detect the irregularityof each cell voltage right after the load is transiently extracted andthe voltage falls down to the lowest voltage. Based on this voltageirregularity, it is possible to determine whether the stationary airflow is appropriate.

The transient-time correction section 61 reduces only the oxygenutilization ratio at the transient time, whereby it is possible toprevent shortage of the oxygen at the transient time and prevent thegenerated voltage from being equal to or lower than the lower limit.When the oxygen utilization ratio at the transient time is to besatisfied, the oxygen is often excessive at the stationary time sincethe voltage drop is large at the transient time. However, by allowingthe transient-time correction section 61 to reduce only the oxygenutilization ratio at the transient time, it is possible to avoid suchwaste of oxygen and to improve fuel consumption.

Further, by allowing the stationary-time correction section 62 to reduceonly the oxygen utilization ratio at the stationary time, it is possibleto prevent shortage of oxygen and stabilize the generated voltage. Ifthe oxygen utilization ratio at the stationary time is to be satisfied,the fuel consumption can be improved. However, the shortage of oxygenoften occurs at the transient time. By allowing the stationary-timecorrection section 62 to reduce only the oxygen utilization ratio at thestationary time, such a defect can be avoided.

When the output of the voltage variation detection section 44 exceedsthe predetermined limit, the erroneous detection prevention section 60increases the oxidizing gas flow or increases the voltage differencebetween the inlet and outlet of the cathode electrode, therebyaccelerating the flow velocity of the oxidizing gas within the cathodeelectrode, blowing off the water remaining in the cathode electrode, anddischarging the water to the outside. By doing so, the water remainingin the cathode electrode can be blown off and discharged to the outside.After eliminating the voltage variation resulting from the waterremaining in the cathode electrode, the low oxygen concentration stateis created. The voltage variation can be thereby detected and erroneousdetection can be prevented.

Each of the oxygen utilization ratio correction sections 64 a and 64 bchanges the parameters for the advancement/delay compensation filter andcorrects the oxygen utilization ratio, whereby it is possible to reduceonly the oxygen utilization ratio at the transient time or thestationary time. By applying the advancement/delay compensation filterto the target air flow, and adjusting the advancement compensation, itis possible to increase only the air flow at the transient time withoutchanging the air flow at the stationary time. Likewise, by adjusting thestationary compensation, it is possible to increase only the air flow atthe stationary time without changing the air flow at the transient time.

Furthermore, by adjusting the delay compensation of theadvancement/delay compensation filter, it is possible to adjust the timerequired until the air flow is stabilized to the stationary value afterthe air flow at the transient time is increased by the advancementfunction, stabilize the air flow to the stationary value by taking along time, and set the state of the low oxygen utilization ratio long.Alternatively, the air flow can be stabilized to the stationary value bytaking a short time, and the state of the low oxygen utilization ratiocan be set short. If the transient load variation frequency is high,then the air flow can be stabilized by taking a long time, the state ofthe low oxygen utilization ratio can be set long, and the voltage dropat the transient time can be reduced.

As can be understood, the second embodiment can provide the fuel cellsystem capable of quickly detecting whether the generated voltage ismade unstable if the air stoichiometric ratio is reduced to near thelimit.

Modification of the Second Embodiment

A modification of the operations of the fuel cell system according tothe second embodiment shown in FIG. 11 will be described with referenceto FIGS. 16 to 18B and 21. The fuel cell system according to thismodification differs from that shown in FIG. 11 in the configuration ofthe voltage variation detection section 44 as shown in FIG. 19. Theother constituent elements of the fuel cell system according to thismodification are equal to those shown in FIG. 11.

As shown in FIG. 21, the voltage variation detection section 44 includesa function storage section 61 which stores a current-to-voltagecharacteristic function of the fuel cell stack 34 with the oxygenutilization ratio used as a parameter, a target load current calculationsection 62 which calculates a target load current from required power, atarget lowest voltage calculation section 63 which inputs the targetload current to the current-to-voltage characteristic function, andwhich calculates a target lowest voltage, a second voltage differencedetection section 65 which detects a difference between the targetlowest voltage and the lowest voltage, a target stationary voltagecalculation section 64 which inputs the target load current to thecurrent-to-voltage characteristic function, and which calculates atarget stationary voltage, and a third voltage difference detectionsection 66 which detects a difference between the target stationaryvoltage and a stationary voltage right after the stationary voltagefalls down to the lowest voltage.

(a) At a stage S10, the response of the load extracted from the fuelcell track 34 is selected similarly to FIG. 12.

(b) At a stage SS15, the target low voltage and the target stationaryvoltage are set. Specifically, the function storage section 61 preparesthe current-to-voltage characteristic function (I-V characteristicfunction) of the fuel cell stack 34 with the oxygen utilization ratioused as a parameter as shown in FIG. 17A. The target load currentcalculation section 62 calculates the target load current from therequired power. The target lowest voltage calculation section 63 and thetarget stationary voltage calculation section 64 input the target loadcurrent to the current-to-voltage characteristic function of the fuelcell stack 34 show in FIG. 17A with the oxygen utilization ratio used asa parameter, and calculate the target lowest voltage and the targetstationary voltage, respectively. It is noted that the required power iscalculated from a throttle opening. In this modification, a functionthat associates the required power obtained by an experiment in advancewith the throttle opening is created and referred to. In FIG. 17A,symbols P₁, P₂, and P₃ correspond to symbols P₁, P₂, and P₃ shown inFIG. 17B, respectively. FIG. 17B shows a voltage locus (time response)by the transient load. The target load current is calculated based onEquation (3).(Target load current)=(Required power[W])÷(Present fuel cellvoltage)  (3)

(c) At a stage SS20, the lowest voltage when the load is transientlyextracted is measured. The second voltage difference detection section65 compares the voltage when the load is transiently extracted and thevoltage falls down to the lowest voltage with the target lowest voltageobtained at the stage SS15, and detects a lowest voltage difference. Ata stage SS30, the voltage right after the load is transiently extractedand the voltage falls down to the lowest voltage is measured. Inaddition, the third voltage difference detection section 66 compares themeasured voltage with the target stationary voltage obtained at thestage SS15, and detects a stationary voltage difference.

(d) At a stage SS40, it is determined whether the lowest voltagedifference measured at the stage SS20 is below a lowest limit. When thelowest voltage difference is below the lowest limit (“NO” at the stageSS40), the processing goes to a stage S90. At a stage S50, it isdetermined whether the stationary voltage difference measured at thestage SS30 is below a predetermined limit. When the lowest voltagedifference is below the lowest limit (“NO” at the stage SS50), theprocessing goes to the stage S90.

(e) At the stage S90, it is determined the water blowoff purging iscontinuously executed a predetermined number of times. When the voltagedoes not recover even after the water blowoff purging is continuouslyexecuted the predetermined number of times (“YES” at the stage S90), itis determined that an abnormality occurs and the processing goes to astage S110, at which “an abnormality alarm” is issued. When the waterblowoff purging is not continuously executed the predetermined number oftimes (“NO” at the stage S90), the processing goes to a stage S100, atwhich the water blowoff purging for blowing off the water remaining inthe air channel of each fuel cell is continuously performed. Next, at astage S105, a variable indicating whether the water blowoff purging iscontinued the predetermined times is counted up. The resetting of thecounter is executed at a stage S95 similarly to FIG. 12.

(f) At a stage SS60, the air flow at the transient time is corrected tobe increased in accordance with the lowest voltage difference measuredat the stage SS20. As shown in FIG. 18A, as the difference between thelowest voltage and the target lowest voltage is larger, the air flow atthe transient time is larger. At a stage SS70, the air flow at thestationary time is corrected to be increased in accordance with thestationary voltage difference measured at the stage SS30. As shown inFIG. 18B, as the difference between the stationary voltage and thetarget stationary voltage is larger, the air flow at the stationary timeis larger.

As stated above, the target load current is input to thecurrent-to-voltage characteristic function of the fuel cell stack 34with the oxygen utilization ratio used as the parameter, the targetlowest voltage is calculated, and the difference between the targetlowest voltage and the lowest voltage is detected. By doing so, it ispossible to detect a voltage variation between the lowest voltage at thetransient time that is a desired and ideal voltage and the lowestvoltage of the fuel cell stack 34 at the present transient time. Basedon this voltage variation, it is possible to determine whether thetransient air flow is appropriate. In addition, it is possible todiscriminate whether the voltage variation is a voltage variation due toa factor that an air utilization ratio is high or a voltage variationdue to the other factor.

Moreover, the target load current is input to the current-to-voltagecharacteristic function of the fuel cell stack 34 with the oxygenutilization ratio used as the parameter, the target stationary voltageis calculated, and the difference between the target stationary voltageand the stationary voltage is detected. By doing so, it is possible todetect a voltage variation between a desired and ideal voltage(stationary voltage) and the present stationary voltage of the fuel cellstack 34. Based on this voltage variation, it is possible to determinewhether the stationary air flow is appropriate. In addition, it ispossible to discriminate whether the voltage variation is a voltagevariation due to a factor that the air utilization ratio is high or avoltage variation due to the other factor.

The entire contents of a Patent Application No. TOKUGAN 2003-279731 witha filing date of Jul. 25, 2003 and a Patent Application No. TOKUGAN2003-324491 with a filing date of Sep. 17, 2003 in Japan are herebyincorporated by reference.

As explained above, the present invention has been described by thefirst and the second embodiments, as well as the modification thereof.However, it should not be understood that the descriptions and thediagrams constituting a part of the disclosure limit the presentinvention. Various alternative embodiments, examples, and operationaltechniques will be apparent to those skilled in the art from the presentdisclosure. That is, the present invention includes those alternativeembodiments and the like, which are not disclosed herein. The presentinvention is therefore limited only by features of the inventionaccording to the scope of the following claims pertinent to thedisclosure.

The invention claimed is:
 1. A fuel cell system comprising: a fuel cellstack configured to generate power by causing hydrogen and oxygen toelectrochemically react with each other; a fuel gas supply deviceconfigured to supply a fuel gas that contains the hydrogen, to the fuelcell stack; an oxidizing agent gas supply device configured to supply anoxidizing agent gas that contains the oxygen, to the fuel cell stack; avoltage detection section configured to detect a voltage generated bythe fuel cell stack; an oxygen concentration transient reduction sectionconfigured to reduce transiently an oxygen concentration in a cathodeelectrode of the fuel cell stack by increasing a load current to beextracted from the fuel cell stack in a response time shorter than aresponse time taken to supply the oxidizing agent gas from the oxidizingagent gas supply device to the cathode electrode; a voltage variationdetection section configured to detect a voltage variation when theoxygen concentration is transiently reduced by the oxygen concentrationtransient reduction section; and a voltage stabilization maintenancedetermination section configured to determine whether a present oxygenutilization ratio is appropriate for maintaining the voltage of the fuelcell stack stable, based on an output of the voltage variation detectionsection.
 2. The fuel cell system of claim 1, wherein the voltagevariation detection section comprises: a lowest voltage measurementsection configured to measure with reference to the voltage detected bythe voltage detection section, the voltage when the oxygen concentrationtransient reduction section transiently reduces the oxygen concentrationand the voltage of the fuel cell stack falls down to a lowest voltage; aprior-to-extraction voltage storage section configured to store avoltage before the load current is extracted; and a first voltagedifference detection section configured to detect a difference betweenthe voltage when the voltage is the lowest voltage and the voltagebefore the load current is extracted.
 3. The fuel cell system of claim1, wherein the voltage variation detection section comprises: a cellvoltage measurement section configured to measure each cell voltage ofthe fuel cell stack right after the oxygen concentration transientreduction section transiently reduces the oxygen concentration; and anirregularity detection section configured to detect an irregularity ofthe each cell voltage based on an output of the cell voltage measurementsection.
 4. The fuel cell system of claim 2, wherein the voltagevariation detection section comprises: a function storage sectionconfigured to store a current-to-voltage characteristic function of thefuel cell stack with an oxygen utilization ratio used as a parameter; atarget load current calculation section configured to calculate a targetload current from required power; a target lowest voltage calculationsection configured to input the target load current to thecurrent-to-voltage characteristic function, and configured to calculatea target lowest voltage; and a second voltage difference detectionsection configured to detect a difference between the target lowestvoltage and the lowest voltage.
 5. The fuel cell system of claim 2,wherein the voltage variation detection section comprises: a functionstorage section configured to store a current-to-voltage characteristicfunction of the fuel cell stack with an oxygen utilization ratio used asa parameter; a target load current calculation section configured tocalculate a target load current from required power; a target stationaryvoltage calculation section configured to input the target load currentto the current-to-voltage characteristic function, and configured tocalculate a target stationary voltage; and a third voltage differencedetection section configured to detect a difference between the targetstationary voltage and the stationary voltage right after the stationaryvoltage falls down to the lowest voltage.
 6. The fuel cell system ofclaim 1, wherein the voltage stabilization maintenance determinationsection comprises a transient-time correction section configured todetermine that a present oxygen utilization ratio is inappropriate forthe maintaining the voltage of the fuel cell stack at a transient timewhen an output of the voltage variation detection section exceeds apredetermined value, and then configured to decrease a oxygenutilization ratio at the transient time.
 7. The fuel cell system ofclaim 1, wherein the voltage stabilization maintenance determinationsection comprises a stationary-time correction section configured todetermine that a present oxygen utilization ratio is inappropriate forthe maintaining the voltage of the fuel cell stack at a stationary timewhen an output of the voltage variation detection section exceeds apredetermined value, and then configured to decrease an oxygenutilization ratio at the stationary time.
 8. The fuel cell system ofclaim 1, wherein the voltage variation detection section comprises anerroneous detection prevention section configured to, when an output ofthe voltage variation detection section deviates from a predeterminedlimit, increases a flow of the oxidizing agent gas or increases apressure difference between an inlet and an outlet of the cathodeelectrode, thereby accelerating a flow velocity of the oxidizing agentgas within the cathode electrode, blowing off water remaining in thecathode electrode, and discharging the water to an outside.
 9. The fuelcell system of claim 6, wherein the transient-time correction sectioncomprises: a target flow calculation section configured to calculate atarget air flow based on a target power generation amount and a targetoxygen utilization ratio; and an oxygen utilization ratio correctionsection configured to correct the oxygen utilization ratio by applyingan advancement and delay compensation filter to the target air flowcalculated by the target flow calculation section, and by changingparameters for the advancement and delay compensation filter.
 10. Thefuel cell system of claim 7, wherein the stationary-time correctionsection comprises: a target flow calculation section configured tocalculate a target air flow based on a target power generation amountand a target oxygen utilization ratio; and an oxygen utilization ratiocorrection section configured to correct the oxygen utilization ratio byapplying an advancement and delay compensation filter to the target airflow calculated by the target flow calculation section, and by changingparameters for the advancement and delay compensation filter.